"The authors focus their attention on the relationship between sunlight and sleep, and take advantage of the fact that the state of Florida, where they conduct their research, is divided into two time zones. The sun comes up an hour later, on the clock, in the Eastern Time Zone than a few miles west in the Central Time Zone, but schools only partially account for this difference when setting their start times, so, on average, students in the Central Time Zone in Florida have more than half an hour more sunlight before school starts than do their counterparts in the Eastern Time Zone, and some have an hour or more additional sunlight, depending on when school starts.

"One might be concerned that people living in different parts of Florida are somehow different in other ways as well, and Heissel and Norris are able to deal with this concern by concentrating on students who moved between time zones, while remaining in the northern part of Florida (typically called the Panhandle). Some students moved between the Eastern Time Zone and the Central Time Zone, thereby gaining extra sunlight in the morning before school, while others moved from the Central Time Zone to the Eastern Time Zone, thereby losing some sunlight before school starts. Their strategy, therefore, is to compare the same students’ test performance before versus after their cross-time zone moves. The authors found that people making these eastward and westward moves in the Florida Panhandle were similar across a large range of characteristics, and tended to follow similar over-time test score trends prior to their moves.

"What happens when children get an extra hour of sunlight before starting school? (The authors estimate the effect of each additional minute of pre-school sunlight, and I’m presenting the effects of a 60 minute difference for ease of explication.) If they are young, math scores are barely affected—the estimated score improvement is just one percent of a standard deviation—but reading scores increase by six percent of a standard deviation. But once they reach puberty (approximately at age 11 for girls and age 13 for boys) math scores improve by eight percent of a standard deviation and reading score improvements remain at six percent of a standard deviation. ...

"Heissel and Norris carried out a thought exercise in which, for every Florida panhandle school district, they assigned the school district’s earliest start times to elementary students, the middle start times to middle school students, and the latest start times to high school students. This calculation would move elementary school start times 22 minutes earlier, middle school start times 13 minutes earlier, and high school start times 44 minutes later, on average.

"Heissel and Norris estimate that making these scheduling switches would raise average math performance by six percent of a standard deviation and average reading performance by four percent of a standard deviation. While not earth-shattering performance changes, they are extremely impressive for a policy change that would cost school districts little to implement – and are approximately one-fourth the difference between an excellent-performing school and an average-performing school."

When I've heard discussions of later start times for high school, several issues usually come up. First, there is an aversion to having the youngest children start earlier. Frankly, this has always seemed a little odd to me. I've got no evidence on the point, but a lot of young children seem to wake up pretty early, and parents who are depending on school for child care have good reason to wish that their younger children were out the door a little sooner. Second, there's an argument that if the high school day starts later, it also needs to end later, which in turn means less time for high school sports and activities, or for after-school jobs. Third, there's an argument that lots of high school students are staying up until the middle of the night as it is, and if they can get up later, they will just stay up later.